General Material Properties

Can get a different perspective of material from different tests on small, medium and large scales depending on which property is most important for the application.

Theory & Mechanism

Mechanism of fire: heat generated from flame → touches the polymer surface → the surface produces volatile fragments, and combust → these are feeded to the heat → develops and creates worse situation

Two general ways to achieve flame resistance:

Solid phase inhibition: by extensive crosslinking at the surface when in presence of heat (form a char) → this can insulate the underlying polymer from flame and prevent further developing

- by Endothermic degradation: Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates. This reaction removes heat from the surrounding, thus cooling the material. The use of hydroxides and hydrates is limited by their relatively low decomposition temperature, which limits the maximum processing temperature of the polymers.

- by Dilution of Fuel: Inert fillers, eg. talc or calcium carbonate, act as diluents, lowering the combustible portion of the material, thus lowering the amount of heat per volume of material it can produce while burning.

- by Thermal shielding: A way to stop spreading of the flame over the material is to create a thermal insulation barrier between the burning and unburned parts. Intumescent additives are often employed; their role is to turn the polymer into a carbonized foam, which separates the flame from the material and slows the heat transfer to the unburned fuel.

Vapor phase inhibition: incorporate materials that when released into flame inhibit the flame (quench the flame) → this means that flame requires increased energy to stay lit since its initial radical reactions are inhibited

- by Dilution of gas phase: Inert gases (most often carbon dioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, and slowing the reaction rate.

- by Gas phase radical quenching: Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide. These react with the highly reactive H* and OH* radicals in the flame, resulting in an inactive molecule and a Cl* or Br* radical. The halogen radical has much lower energy than H* or OH*, and therefore has much lower potential to propagate the radical oxidation reactions of combustion. Antimony compounds tend to act in synergy with halogenated flame retardants. The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.

Both methods of inhibition used in typical flame-resistant materials<math>^1</math>

Making Fire-Retardant Fabrics

Two ways to make<math>^1</math>:

Additive incorporation
- Less expensive method

- Use commodity high volume plastics and add components that improve fire-retardant characteristics

- Used for polymers needed in:
Extreme temperatures (600-1000oC) for a few minutes/ Moderate temperatures (200-300oC) in air for longer periods of time

- Example: Nomex®

- Limitations

1. Gas phase radical quenching: The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.

2. Some fire-retardant polymers, such as polyimides have very low solubility in all solvents → makes polymer intractable and difficult to process
(This is due to its linear sequence of cyclic structures that is thermally stable up to about 800 C when it begins to char.)

- Some health hazards exist but for the most part, the safety of the polymers are tested before they are incorporated in garments.

a. In 1970’s, one type of Brominated Fire Retardant (Tris-BP), used in clothing at the time, was found to be mutagenic and nephrotoxic. It’s production was quickly stopped.

b. In another incident in the 1970s , polybrominated biphenyls were removed from the market because of poisonings in Michigan attributed to the inadvertent mixing of a bag of Firemaster FF-1, a commercial PBB mixture, into animal feed. The contamination of animal feed resulted in loss of livestock and long-term impacts on the health of farm families in Michigan.